Water is an unavoidable contaminant in fuel. It can affect fuel system component reliability and lead to operational delays and increased maintenance activities. In addition, the propensity for microbiological contamination is directly proportional to the presence of water and the temperature within fuel tanks. Sources of water in aircraft fuel tanks is from fuel loaded into the aircraft fuel tanks during refuel (dissolved water) and from air entering the aircraft fuel tanks via its ventilation system. It is estimated that up to 30-40% of water in the fuel of aircraft fuel tanks is currently entering via the ventilation system, depending on atmospheric conditions.
During refill while the aircraft is on the ground, fuel is loaded onto the aircraft at a high volume flow rate to reduce the turnaround time. The fuel displaces the air in the fuel tanks as the tanks fill up. The air is pushed out of the fuel tanks via the ventilation system.
During climb, due to decreasing ambient pressure as the aircraft ascends, air expands in the ullage. Although the engines are consuming fuel, the rate of fuel consumption is less than the rate of air expansion. This results in a net outflow of air through the ventilation system.
During cruise, the fuel level decreases steadily as the engines consume the fuel. A decrease in the fuel level causes an increase in the ullage volume, and excess air is drawn in from ambient via the ventilation system to equalise pressures. At cruise, the ambient air is relatively cold and dry.
During descent, due to increasing ambient pressure as the aircraft descends, air contracts in the ullage. This results in a net inflow of ambient air through the ventilation system. The ingress of ambient air during descent brings relatively warm, humid air into the fuel system, and hence a significant volume of water enters the fuel tanks via the ventilation system.
There is therefore a need in the art for a system and method for reducing the ingress of moisture into an aircraft fuel tank via its ventilation system.